With the development of modern oil and gas drilling technology, measuring while drilling tool (MWD tool) is more and more widely used in the drilling process. The MWD tool transmits the underground data to the ground by means of mud pulse, electromagnetic wave, or sound wave, so that the technicians on the ground can analyze the data and then adjust the drilling progress accordingly.
In the prior art, power is supplied to a downhole MWD tool mainly in two ways, namely through battery pack and through generator. Because the capacity and safety of a battery pack are greatly affected by the temperature, when the temperature reaches 120° C., the capacity of the battery pack decreases by 20%. The temperature limit of a battery pack is about 175° C. In addition, the transducer and electronic circuits of the MWD tool only require a few or a dozen watts of power, however, part of the underground measuring and controlling system can consume as much as 700 watts. To prolong the operation time of the tool underground, downhole generator is mainly used as the power source for the MWD tool at present, which supplies power for the battery and/or the transducer group and the signal generating device.
U.S. Pat. No. 5,517,464 discloses an MWD tool which integrates a mud pulse generator and a turbine generator. The turbine generator comprises a turbine impeller, a drive shaft, a transmission, a three-phase alternator, and a rotational speed measurement device. Because the space underground is limited and the generator can only provide relatively low power, the turbine generator cannot meet the requirement of the drilling process. In addition, in this device, a gearbox is used to obtain the rotary speed response from the turbine and the generator, which adds complexity to the structure of the MWD tool. Moreover, since the coils directly contact the mud, it requires highly of the mud quality, bearing performance, and the insulation of the coils; and the coils are easy to be damaged at high speed under severe environment, such as high temperature and intense vibration, for long terms.
CN 201010533100.2 discloses a petroleum drilling mud generating system which comprises coil windings, a magnet, an impeller, an upper plug, a lower plug, a central shaft, and an isolation sleeve, wherein the magnet is embedded in the impeller hub; the coil windings are fixed in a closed cavity formed by the central shaft, the upper and lower plugs, and the isolation sleeve; and the impeller hub is in clearance fit with the isolation sleeve. When the mud with pressure flushes from top to bottom, the flushed impeller rotates so that the magnet embedded in the impeller hub rotates synchronously with the impeller, and the coils cut through the magnetic lines of force to generate power. Moreover, an abrasion-resistant alloy sleeve is provided between the impeller and the isolation sleeve, which provides supporting and straightening functions when the impeller rotates. And a shock absorber is provided between the alloy sleeve and the plugs, so as to reduce influence of the mud impact on the abrasion-resistant alloy sleeve.
This petroleum drilling mud generating system is advantageous in that it no longer uses dynamic seal. However, it adopts clearance fit between the rotor and the isolation sleeve, with mud as the lubricant, so as to fulfill the functions of supporting and straightening. When operating at high speed in the mud, because sand unavoidably exists in the mud, sand stuck can easily occur, causing the whole system to fail and mud lubrication failure. In addition, the metal isolation sleeve, which is placed between the magnet and the coil windings, suffers from eddy current loss in a changing magnetic field, making it very difficult for the system to generate high power. In the meantime, eddy current loss directly manifests as heat, causing temperature rise.